In a conventional lock, a key is inserted into a key cylinder to rotate a driving arm which, in turn, displaces a security bolt into a locking recess in a door frame to lock the door to the frame.
Such a lock may also include a spring bolt normally urged outwardly from the door, to act as a cam that is retracted when the door is closed.
Also known are locks that are locked by a key from the inside or outside. Such locks employ a double cylinder operable by the same key. Also known are locks that require the use of a key only from outside, while from the inside they are opened freely by turning a knob or handle whose angular displacement produces directly retraction of the bolt and as soon as the door is closed, a spring returns the bolt automatically to its projected locking state. The locks have the advantage that, in case of emergencies such as a fire, for example, they can be opened without a key. However, they have the disadvantage that the door can be easily opened by children or the mentally handicapped without supervision.
Other problems that are usually found in conventional locks with free opening from the inside is that, each time a person wishes to go out even for a few moments, the person must take the precaution of taking the key to get back in as, otherwise, the door might close accidentally (for example, due to the wind) leaving the person locked out.
Since the key cylinder of the lock must have its key opening accessible from the outside, it then results that the simplest and commonest way of destroying such a lock is by directly forcing the cylinder by means a perforating instrument or the like. There are disadvantages in the installation of the locks which make the system more expensive.
Thus, for example, not all the doors have the same distance between its door edge and the door frame and all the locking means may not have enough depth so as to easily engage the locking bolt which requires the installers to take measures and adjustments, with the consequent loss of time.
Another disadvantage is that all doors do not open from the same side; and since the bolt of the lock has one face that operates as a cam while the other face operates as a locking means, it is necessary to manufacture locks for left opening, and right opening doors with the corresponding production complications.